Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit of a fine pattern on e.g. a Si substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices has been accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½, of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼, of the exposure wavelength, and even if an immersion method of ArF laser (wavelength: 193, nm) is employed, about 45, nm is presumed to be the limit of the exposure wavelength. From this point of view, EUV lithography, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser, is expected to be prospective as a next generation exposure technique for 45, nm or below. In this specification, EUV light means a light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically a light ray having a wavelength of from about 10, to 20, nm, particularly about 13.5, nm±0.3, nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive indices of substances at such a wavelength are close to 1,, whereby it is not possible to use a conventional dioptric system like photolithography employing visible light or ultraviolet light. For this reason, for EUV lithography, a catoptric system, i.e. a reflective photomask and a mirror, is employed.
A mask blank is a stacked structure for production of a photomask, which has not been patterned yet.
In the case of an EUV mask blank, it has a structure wherein a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, are formed in this order on a substrate made of e.g. glass. On the absorber layer, as a case requires, a low reflective layer for pattern inspection wavelength (from 190, to 260, nm) is formed. As the absorber layer, a material having a high extinction coefficient for EUV light, specifically, for example, a material having Ta as the main component, is used. As the low reflective layer, a material having low reflection properties for pattern inspection wavelength, specifically, a material containing Ta and O as the main components is used.
To produce a photomask from the mask blank of the above structure, a resist film is applied on the uppermost layer (the absorber layer, or in a case where a low reflective layer is formed on the absorber layer, the low reflective layer) of the mask blank. On the resist film, a pattern is formed by an electron beam lithography apparatus, and then using the resist film having a pattern formed thereon as a mask, an etching process is carried out, whereby the pattern is transferred to the absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer). Here, since the resist film is also consumed in the etching process, the resist film should be sufficiently thick. The thickness of the resist is usually at a level of 150, nm although it depends on the type of the resist and the etching conditions.
In recent years, while miniaturization and high densification of the pattern are in progress, a pattern with a higher resolution has been required, and in order to obtain a pattern with a high resolution, the resist film is required to be thin.
However, if the resist film is made thin, since the resist film is consumed in the etching process, the accuracy of the pattern transferred to the absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer) may be lowered.
To overcome the above problem, it has been commonly known that the resist film can be made thin by providing a layer (hard mask layer) made of a material having resistance to the etching conditions for the absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer) (Patent Documents 1, and 2). That is, the resist film can be made thin by forming such a hard mask layer to increase the etching selectivity of the absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer) to the hard mask layer under etching conditions for absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer), specifically, the ratio of the etching rate for the absorber layer (in a case where a low reflective layer is formed on the absorber layer, the etching rate for the absorber layer and the low reflective layer) to the etching rate for the hard mask layer under etching conditions for absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer).
Hereinafter in this specification, “the etching selectivity under etching conditions for absorber layer” means the etching selectivity of the absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer) to the hard mask layer under etching conditions for absorber layer (in a case where a low reflective layer is formed on the absorber layer, the absorber layer and the low reflective layer), and the etching selectivity can be determined by the following formula:Etching selectivity=(the etching rate for the absorber layer (in a case where a low reflective layer is formed on the absorber layer, the etching rate for the absorber layer and the low reflective layer))/(the etching rate for the hard mask layer)
Patent Document 1, discloses that a material containing chromium (Cr), zirconium (Zr) or indium (In) as the main component is preferred as a material constituting the hard mask layer, whereby obtainable hard mask has high etching resistance to fluorine type gas plasma to be used for etching of a light absorber layer (absorber layer) containing Ta as the main component, and it can readily be etched by means of chlorine type gas plasma.
Patent document 2, discloses that a layer containing any of silicon, oxygen, carbon and chromium is preferred as a hard mask layer, which has high etching resistance in a dry etching process based on fluorine.